The present invention relates to piezoelectric sintered ceramics and piezoelectric elements constituted by such piezoelectric sintered ceramics suitable for actuators and laminate-type piezoelectric transformers which are parts of inverter circuits for cold-cathode discharge tubes, back-lights of small liquid crystal displays, etc.
Because liquid crystals used in liquid crystal displays do not light themselves, back-light systems having discharge tubes such as cold-cathode discharge tubes are generally mounted rear or either lateral side of liquid crystal displays. To drive the cold-cathode discharge tubes, AC voltage of several hundreds of volts or more is usually required, though actual voltage level may vary depending on the length and diameter of the discharge tubes used. Japanese Patent Laid-Open No. 5-114492 discloses an inverter comprising a piezoelectric ceramic element (piezoelectric transformer) as means for generating such a high AC voltage.
This piezoelectric transformer has an extremely simple structure because wire winding is unnecessary, whereby it can be miniaturized, made thin and low in production cost. The structure and function of such a piezoelectric transformer will be described referring to FIG. 1. FIG. 1 schematically shows a Rosen-type piezoelectric transformer proposed by C. A. Rosen in 1956.
In FIG. 1, 1 represents a piezoelectric ceramic made of, for instance, PbTiO.sub.3 --PbZrO.sub.3 (PZT). This piezoelectric transformer is coated with a pair of silver input electrodes 3, 4 on both upper and lower surfaces on the left side to form a driving portion and a silver output electrode 5 on the right side to form an electricity-generating portion. The piezoelectric transformer is polarized in a thickness direction in the driving portion in the left half and in a longitudinal direction in the electricity-generating portion in the right half, as indicated by the arrows A and B, respectively.
When AC voltage having substantially the same frequency as that of resonance frequency in the longitudinal direction of the piezoelectric ceramic 1 is applied between the input electrodes 3, 4, strong mechanical vibration occurs in the piezoelectric ceramic 1 in the longitudinal direction. As a result, electric charge is generated due to piezoelectric effects in the electricity-generating portion in the right half, whereby output voltage V.sub.o is generated between the output electrode 5 and one of the input electrodes, for instance, an input electrode 4.
Voltage step-up ratio (V.sub.o /V.sub.i), wherein V.sub.i is input voltage, achieved by the piezoelectric transformer having the above structure is expressed by the formula (1): EQU (V.sub.o /V.sub.i)=A.multidot.k.sub.31.multidot.k.sub.33.multidot.Q.sub. M.multidot.L/T (1),
wherein k.sub.31 is an electromechanical coupling coefficient in transverse effect, k.sub.33 is an electromechanical coupling coefficient in longitudinal effect, Q.sub.M is a mechanical quality coefficient, L is the length of the piezoelectric transformer, T is the thickness of the piezoelectric transformer, and A is a constant. k.sub.31, k.sub.33, and Q.sub.M are coefficients determined by the piezoelectric materials per se, and L and T are determined by the size of the piezoelectric ceramic element.
Because the piezoelectric transformers used for the above-described back-lights are required to provide as high AC voltage as several hundreds of volts or more, they should have a high voltage step-up ratio. For this purpose, it is effective to make the piezoelectric transformers as thin as possible and/or as long as possible, as is clear from the formula (1). However, their thickness and length are inevitably limited from the viewpoint of mounting space and mechanical strength.
To solve these problems, Japanese Patent Laid-Open No. 7-302938 discloses a laminate-type piezoelectric transformer constituted by laminating thin piezoelectric ceramic sheets and connecting their driving portions in parallel. FIG. 2 schematically shows such a laminate-type piezoelectric transformer in which driving portions are laminated with internal electrodes 2 alternately and connected to the input electrodes 3, 4 in parallel. An output electrode 5 is attached to one longitudinal side of the laminate-type piezoelectric transformer.
The above-described laminate-type piezoelectric transformer constituted by laminating thin piezoelectric ceramic sheets 1 and internal electrodes 2 alternately can generally be produced in the same manner as laminate-type ceramic capacitors. Namely, a mixture of metal oxides of lead, zirconium, titanium, etc. is calcined and formed into green sheets each generally having a thickness of 50-150 .mu.m. A precious metal paste is applied to a surface of each green sheet, for instance, by a screen printing method to form an input electrode 2. A plurality of electrode-coated green sheets are laminated, pressed into an integral body, and then sintered.
Precious metals used for the internal electrodes 2 of such laminate-type piezoelectric transformers should not be oxidized and melted at sintering temperatures, and they preferably are inexpensive. Thus, Ag--Pd alloys are generally used as internal electrode materials for the laminate-type piezoelectric transformers as in the case of the laminate-type ceramic capacitors. Ag--Pd alloys have as high melting points as 1554.degree. C., for instance. The higher the percentage of Pd, the higher temperature the Ag--Pd alloys can withstand at sintering. However, because Pd is easily changeable in volume by oxidation and reduction during the sintering, too much Pd would be likely to cause the peeling (delamination) of piezoelectric ceramic sheets during the sintering. On the other hand, if the percentage of Pd were too small, the Ag--Pd alloys would have too low melting points, whereby the sintering temperatures of the piezoelectric ceramics should be decreased.
In the laminate-type ceramic capacitors, an Ag--Pd alloy having an Ag/Pd weight ratio of 70/30 is predominantly used as internal electrode materials. However, if this alloy is used for the laminate-type piezoelectric transformers, the piezoelectric ceramics should be sintered at temperatures of about 1100.degree. C., which are much lower than the conventional sintering temperatures of about 1250.degree. C. There have never been conventional piezoelectric ceramics that can be sintered at temperatures of about 1100.degree. C.
According to Jpn. J. Appl. Phys. vol. 34, pp. 5270-5272 (1995), the laminates of conventional piezoelectric ceramics with internal electrodes of Ag--Pd alloys can be sintered in the air at 1100.degree. C., about 100.degree. C. lower than the sintering temperatures of the piezoelectric ceramics alone, due to the sintering acceleration function of the internal electrodes, thereby providing laminate-type piezoelectric ceramic vibrators excellent in piezoelectric properties. However, in the resultant laminate-type piezoelectric ceramic vibrators, crystal grains in portions near the internal electrodes grow too much, while portions distant from the internal electrodes have extremely uneven crystal structures having small crystal grains and insufficient density.
In the uneven crystal structures of the piezoelectric ceramics, pores and cracks are likely to be generated along crystal grain boundaries, resulting in small mechanical strength. Also, because the mechanical strength of the piezoelectric ceramics is inversely proportional to the crystal grain sizes thereof, the piezoelectric ceramic portions near the internal electrodes are weaker and more brittle than those distant from the internal electrodes. When piezoelectric ceramic elements are constituted by such conventional piezoelectric ceramics, higher input voltage leads to heat generation and decrease in voltage step-up ratio due to increase in internal loss, and mechanical breakage at the time of polarization and driving. Thus, such piezoelectric ceramic elements do not deserve practical use.